Fluoropolymers are non-reactive plastics that are excellent insulators and are commonly used as exterior coatings on electrical wires. One particularly common fluoropolymer used to coat wires is poly(tetrafluoroethylene) (PTFE). Fluoropolymer wires can be used in demanding applications, such as aerospace, because they are chemically resistant. Additionally, PTFE is flame resistant and other fluoropolymers will only smoke rather than burn. Electrical wires may be labeled with some indicia printed on the exterior of the insulation, such as the manufacturer's name, lot number, product name, technical specification of the wire, or a colored identifying stripe. Thus, it is often necessary to print appropriate labeling on the wire prior to installation. However, many inks do not adhere sufficiently to the surface and can be worn away under normal use conditions.
Typically, such indicia are printed on fluoropolymer coatings with either solvent- or water-based contact inks using all-wheel printers (e.g., contact printers, bandmarkers, and off line printers). Some examples of known contact inks include the GEM 6000 series, the GEM 1100 series, and the GEM 1150 series, all manufactured by Gem Gravure Co., Inc. (West Hanover, Mass., USA). While many contact inks adhere to the PTFE, the use of all-wheel printers can have limitations. For example, the print wheel contacts the wire, and the pressure of this contact can damage the wire. Additionally, the printing wheels are laser engraved, so changing the message requires replacing the print wheel, which is expensive.
Thus, it is desirable to use a continuous inkjet printer to print indicia on a wire because the messages can be easily changed, the wire is not damaged by the pressure of the wheel, and less ink is required.
Continuous inkjet printing is a type of non-contact printing method where a conductive fluid is ejected under pressure through a small nozzle after it is perturbed by sonic waves from a vibrating piezo crystal. Common nozzle sizes are 42 micron, 55 micron, 60 micron, and 70 micron. The sonic waves cause the ink stream to separate into drops as the stream exits the nozzle, which are then charged. Different charges result in different drop placements as the drops are deflected by an electromagnetic field, thereby creating a pattern that forms an image. If the ink is not conductive, then the message can't be formed. Thus, a continuous inkjet printer requires conductive ink. Continuous inkjet printing differs from thermal inkjet printing, which is more commonly found in consumer and office inkjet printers. In a thermal inkjet printer, a pulse of current passes through a heating element to vaporize the ink to form a bubble, which is then propelled onto the target substrate. In contrast to continuous inkjet ink printers, thermal inkjet printers do not require a conductive ink formulation and are typically not suitable for use in a continuous inkjet ink printer.
In order for an ink to be used in an continuous inkjet printer, it should meet several characteristics. The viscosity should range between 1 cP (centipoise) to 10 cP at 25° C. The ink should, generally, have a conductivity value of at least 200 μS/cm. More preferably, the ink should have a conductivity value of at least 500 μS/cm. Even more preferably, the ink should have a conductivity value of at least 750 μS/cm. One of skill in the art will recognize that the conductivity value depends on the printer in which the ink is used. The ink also should be capable of passing through the filters of the printer. For example, a 15 mL sample of ink should be able to pass through a 2.7 micron WHATMAN GF/D filter in less than 5 minutes under an applied pressure of 10 inches of mercury (inHg). The ink typically should also separate into drops when agitated with a Piezo-electric crystal or drop-generator at about 1 kHz to about 200 kHz.
While PTFE dispersions can be used to coat metal, those dispersions are primarily found in aqueous compositions. However, solvent-based inks are generally preferred because they dry more quickly than aqueous-based inks Examples include U.S. Pat. Nos. 4,139,576; 4,400,319; 4,548,986; 5,030,667; 5,168,013; 5,168,107; 5,188,764; 5,230,961; 7,462,667; 7,671,112; and 8,222,322. There are, however, some known examples of PTFE dispersions in solvent. Examples include U.S. Pat. Nos. 3,352,811; 3,631,141; and 6,071,564.
While the PTFE dispersions in solvent have been used in wire marking, they typically are applied with a gravure wheel or other similar printing method and are not suitable for inkjet printing for at least two reasons. First, the drops may not separate sufficiently in an inkjet print head. Second, the particle size may be too large, which can cause the inkstream to wander and the nozzle to clog. In the inkjet printhead, once the stream of drops exits the nozzle, the stream needs to hit a consistent location on a solvent recovery device, which is called a gutter. The gutter sucks the drops back into the printer, so that the ink can be reused. If a large particle passes through the nozzle, but is not big enough to clog it, the pressure can change, causing the ink stream to move, which creates a mess and can shut down the printer. To prevent inkstream wandering and nozzle clogging from occurring, most inkjet manufacturers place a filter before the print head. If the filter clogs, then the printer will not operate.
Known ink jet inks often are unable to print on fluoropolymer surfaces for several reasons. For example, typically they do not sufficiently bind to fluoropolymer surfaces in order to withstand typical abrasive forces. They also tend to fail due to poor wetting ability and poor heat stability. Some examples of inkjet inks that are generally not suitable for printing on PTFE and other fluoropolymer surfaces include U.S. Pat. Nos. 7,465,759 and 7,432,316. One example of an inkjet ink with a fluorinated resin is U.S. Patent Publication No. 2011/0039077. This ink, however, may not be suitable for continuous inkjet printing because it is not sufficiently conductive.
Thus, there is a need for an improved inkjet ink formulation having improved adhesion to PTFE and other fluoropolymer surfaces.